Large industrial electromagnet machines, such as large electrical generators used in power plants operate at extremely high energy levels. For example, large steam powered generators may operate at voltages on the order of 20,000 VAC and currents on the order of 20,000 Amps AC. The electrical power from the generator is typically carried to high potential step-up transformers by a power conductor structure comprising three large copper bus structures that are isolated from each other and are commonly known as isolated phase, or isophase, bus structures. Each isophase bus structure includes flexible connections, formed by flexible copper straps, located at spaced intervals along the bus structure. For example, there may be a flexible strap connection located every six to twenty feet along the bus structure. The flexible connections typically comprise multiple heavy flexible copper straps having opposing ends that are bolted into place on adjacently located sections of the bus structure. The flexible copper straps accommodate thermally induced expansion of the bus structures that can result from the high current transported through these structures. Large steam powered units may have up to forty flexible copper straps per flexible connection between the adjacent sections of the bus structure.
Temperature changes along the isophase bus structure, such as may result from changes in the power conducted through the bus structure, can cause the bolts at either end of the flexible straps to loosen as the bus structures expand and contract. A loose connection at the interface between the flexible straps and the adjacent sections of the bus structure will result in decreased current flow in the loosened strap, causing an increase in the temperature of the remaining properly attached straps as they carry the current of the loosened strap, potentially resulting in further loosening of the bolts connecting the flexible straps.
The flexible strap connections are enclosed in an isophase bus shell that surrounds the bus structure and is typically 30 to 60 inches in diameter. Accordingly, observation of the flexible connections is obstructed by the bus shell, making it difficult to monitor the condition of the flexible connections. A final failure of the bus structure generally occurs suddenly, with little or no warning since operation with only a few of the flexible straps securely in place may be sufficient to avoid or minimize a voltage drop across the flexible connection until the failure of the bus structure occurs. In particular, if a failure of the straps progresses to the end, there may be a big electrical arc that blows out the section of the isophase bus structure at the location of the strap failure. The resulting ground fault will trip the generator. However, current will continue to flow through the bus, providing energy to support the arc, until the large amount of energy in the generator rotor field has decayed and been converted to heat. Replacement parts to repair such a failure are often difficult to obtain quickly, such that a bus failure may result in an extended unplanned outage.
It is known to provide an infrared monitoring device for measuring the temperature of a flexible connection on an isophase bus. For example, such an infrared monitoring device comprised an open aperture sensor, such as a thermopile, where the geometry of the aperture provided a wide field of view. Only a small portion of the field of view for the aperture included the flexible straps, and a relatively large portion of the field of view included the inner surface of the bus shell. Accordingly, changes in the temperature of the inner surface of the shell, which have a substantially higher energy output than the flexible straps, produce a greater effect on the readings of the monitoring device than even large changes in the temperature of the flexible straps.
In an alternative implementation of the above-described monitoring technique, an infrared lens is positioned at the aperture to narrow the field of view seen by the sensor, and thus limit the field of view to the area of the flexible straps. Although addition of the infrared lens has been effective to narrow the field of view to monitor the flexible straps, there are still negative factors associated with this technique including the cost of the infrared lenses significantly increasing the expense of the system. In addition, infrared lenses generally have a wavelength band where they do not pass infrared radiation, and the lens provides an exposed optical surface that may collect dirt and/or dust to cause an erroneous low temperature measurement due to a reduction in the energy transmitted through the lens to the sensor. Further, even with the addition of the infrared lens, the monitoring device still only provides an average temperature measurement made across all of the straps located in the field of view of the lens, rather than enabling temperature measurements of the individual straps in the connection.